Interpretive Summary: This case report describes the pathological, microbiological, antifungal susceptibility and molecular genetic characterization of a filamentous fungal pathogen responsible for the mass mortality of 152 lined seahorses in a public aquarium in Atlanta, Georgia. Autopsies of seahorses exhibiting skin lesions revealed that their death was due to highly invasive septate fungal hyphae in vessels and major organs throughout the fish. Cultures of diseased tissue yielded a mold that was identified as a member of the genus Fusarium because it produced diagnostic canoe-shaped spores. Analyses of DNA sequence data from portions of three genes showed that the seahorse pathogen was an unnamed species within the Fusarium solani species complex designated FSSC 12-a Fusarium sp. All known isolates of this species appear to be pathogenic to diverse marine animals. The seahorse isolate was resistant to a wide spectrum of antifungal drugs. Findings reported in this study should be of interest to clinical microbiologists and veterinarians charged with diagnosing and treating fungal infections in animals and to conservation biologists who are trying to bring this species back from the brink of extinction.

Technical Abstract:
During a 4 month epizootic, 100% of 152 lined seahorses Hippocampus erectus in three separate groups died while in quarantine following shipment to a public aquarium. Twelve animals with skin depigmentation and ulceration were received by the Aquatic Pathology Service, University of Georgia, College of Veterinary Medicine for diagnostic evaluation. Microscopically, lesions in 11 seahorses included multifocal epithelial necrosis and ulceration associated with 2-7 µm diameter, branching, septate fungal hyphae, typically accompanied by deeper infiltration into underlying skeletal muscle. Angioinvasion, with vascular thrombosis and tissue infarction was a prominent feature in multiple animals. Fungal invasion of one or more internal organs was observed in four. Hyphae appeared to course freely through tissues and elicited little or no inflammatory response. Fusariosis has been reported sporadically in fish and other aquatic organisms, but identification has often been limited to the genus level based solely on morphologic features. Morphologic characteristics of the fungus isolated from this case were consistent with the Fusarium solani species complex (FSSC), which includes over 50 members that can only be identified definitively using DNA sequence data. A three-locus typing scheme identified the isolate as a distinct species/haplotype, designated FSSC 12-a, belonging to a specific lineage that appears adapted to aquatic environments and disease in marine animals. Empirical treatment with itraconazole failed to stop mortalities and subsequent in vitro antifungal susceptibility data demonstrated a lack of clinical efficacy for this agent. Effective treatment in human medicine has similarly been limited by poor susceptibility to several classes of antifungal compounds.